core/cgroup: fix TasksMaxScale percentage serialization (#41011)

core/cgroup: fix TasksMaxScale percentage serialization (#41011)

bus_cgroup_set_tasks_max_scale() used a hand-rolled percentage format
that produced values ~10x too small (e.g., "TasksMax=4.0%" instead of
"TasksMax=40.00%").

On daemon-reload, the incorrect value was re-read, silently reducing
the effective TasksMax by ~10x and causing fork rejections on systems
with high thread counts.

Fix by using the existing PERMYRIAD macros, consistent with memory
property handlers (MemoryMax, MemoryHigh, MemoryLow, etc.).

Fixes: #41009